1996-02-01

Modeling the Effects of Intake Generated Turbulence and Resolved Flow Structures on Combustion in DI Diesel Engines 960634

Previous studies have shown the importance of the in-cylinder flow field which exists prior to fuel injection on performance and emissions behavior of direct injected (DI) diesel engines. Key parameters in the flow field are the turbulence level and the resolved structures, such as swirl and tumble flow. These characteristics are known to have significant effects on the fuel vaporization, droplet break-up, and fuel-air mixing. The relative importance of these effects is investigated through simulation of injection into a stirred, heated, constant volume combustion bomb, using the computational fluid dynamics codes KIVA-3 [9] and KIVA-II [10]. Initial conditions for these simulations are based on in-cylinder conditions which exist in a heavy duty DI diesel engine immediately prior to fuel injection.
A parametric study of the effects of swirl flow and pre-injection turbulence level on the fuel vaporization, mixing and combustion processes is carried out, based upon variations of the baseline values. Several quantities are calculated in order to better characterize the influence of the pre-injection flow field on the combustion. Included in these are the standard deviation of oxygen concentration, the fuel vapor filling volume, and the ignition delay time.
The effects of the intake flow field are further investigated by varying the geometry of the intake ports and intake runners in a model dual port DI diesel engine in KIVA simulations. Four different intake geometries are evaluated, using similar quantities to those computed for the combustion bomb. In addition, the persistence of resolved flow structures is studied through comparison of various macroscopic length scales. The effect of the intake flow on mixing before and during the combustion events are related to the relative levels of particulate and nitrous oxide emissions.

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